When a pressure sensitive adhesive is used to adhere an optical film, such as a polarizing plate and a retardation film, to a liquid crystal panel and the like, or to adhere a protective film to an optical film, problems such as wrinkles, air bubbles, entrapment of foreign substances, and misalignment can occur. In such a case, the adhered optical film or protective film may be peeled off and re-adhered or the optical film may be peeled off so that the expensive liquid crystal panel can be recovered and recycled. In view of this, pressure sensitive adhesives for optical films that are used to adhere an optical film to an adherend, such as a liquid crystal panel, or to adhere a protective film to an optical film, need to be capable of being peeled at a suitable peeling strength without leaving an adhesive residue, and to have reworking properties that allow the film to be re-adhered.
Further, pressure sensitive adhesives for optical films also need to have a high durability, in which air bubbles are not produced and nor does the film peel from the adherend even when exposed to heating or wet heat.
In particular, pressure sensitive adhesives for optical films used under conditions which are harsher than normal, such as in an optical film that is used in a large-scale display, an automobile display or monitor, an outside display and the like, not only need a much higher adhesive strength than a conventional one, but must also have better reworking properties and durability.
Examples of pressure sensitive adhesives for optical films that have been proposed include:    (1) A pressure sensitive adhesive for a polarizing plate or a retardation plate that includes a graft copolymer or a block copolymer which contains a high glass transition temperature (Tg) polymer segment having a Tg of 50° C. or more and a molecular weight of 500 to 1,000,000 and a low-Tg polymer segment having a Tg that is at least 70° C. lower than the high-Tg polymer segment, wherein the molecular weight as a whole is from 400,000 to 2,000,000 (refer to Patent Document 1); (2) A pressure sensitive adhesive for an optical film that contains 100 parts by weight of an acrylic polymer with a weight average molecular weight of 1,000,000 or more and 1 to 40 parts by weight of an acrylic oligomer with a glass transition temperature of −5° C. or less and a weight average molecular weight of 800 to 50,000 (refer to Cited Document 2); and (3) A pressure sensitive adhesive for an optical film that includes an acrylic polymer as a main component which preferably has a weight average molecular weight of 1,000,000 or more and an acrylic oligomer formed from a block copolymer which has a polymer block with a glass transition temperature thereof alone of −5° C. or less (refer to Cited Document 3).
However, Patent Document 1 only specifically describes as a working example a pressure sensitive adhesive formed from a graft copolymer having a high-Tg polystyrene segment as a branch and a butyl acrylate polymer segment as a main chain. With the pressure sensitive adhesive described in Patent Document 1, a chemical crosslinking treatment is required in order for the adhesion properties to be exhibited. Further, to carry out the crosslinking, functional groups such as hydroxyl groups and carboxyl groups are introduced in advance into the low-Tg polymer segment (main chain) constituting the graft copolymer, which acts as the base of the pressure sensitive adhesive, and a crosslinking agent (e.g., “Coronate L”, a trifunctional isocyanate crosslinking agent) is added to a solution of the pressure sensitive adhesive during the coating of the pressure sensitive adhesive, so that the graft copolymer acting as the base is made to undergo chemically crosslinking. Consequently, the pressure sensitive adhesive described in Patent Document 1 requires the post-treatment step of chemical crosslinking during production of the pressure sensitive adhesive type optical film, so that productivity is reduced and adhesion performance tends to vary due to uneven crosslinking.
Further, in the pressure sensitive adhesives described in Cited Documents 2 and 3, the weight average molecular weight of the acrylic polymer acting as the pressure sensitive adhesive base is 1,000,000 or more, which is very high. Consequently, the solution viscosity is high, so that to obtain a pressure sensitive adhesive solution having excellent coating properties and a low viscosity, a large amount of organic solvent has to be used to lower the solid concentration of the pressure sensitive adhesive solution. The use of large amounts of organic solvent produces problems such as environmental contamination and an increase in the time required for the solvent removal step after coating. Moreover, similar to the pressure sensitive adhesive described in Cited Document 1, the pressure sensitive adhesives described in Cited Documents 2 and 3 also require carboxyl groups and the like functional groups to be introduced into the acrylic polymer acting as a base, which react with a crosslinking agent (e.g., “Coronate L”, a trifunctional isocyanate crosslinking agent) to form chemical crosslinks, whereby the adhesion properties are exhibited. Since the chemical crosslinking is carried out by adding a crosslinking agent to an pressure sensitive adhesive solution during coating of the pressure sensitive adhesive, a separate post-treatment step of chemical crosslinking during production of the pressure sensitive adhesive type optical film is required, which reduces productivity. In addition, adhesion performance tends to vary due to uneven crosslinking.
Although hot melt pressure sensitive adhesives which include an acrylic triblock copolymer are known (refer to Patent Documents 4 and 5), these documents do not disclose the use of those hot melt pressure sensitive adhesives in an optical film. Further, these documents also do not disclose that the hot melt pressure sensitive adhesives can be turned into a solution type pressure sensitive adhesive by dissolving in an organic solvent, instead of melting, for use as an optical film.
Under such circumstances, the present inventors developed a non-chemical crosslinking type pressure sensitive adhesive for an optical film which has a specific acrylic triblock copolymer as a main component, which they have filed a patent application for (refer to Patent Document 6). This non-chemical crosslinking type pressure sensitive adhesive for an optical film developed by the present inventors does not suffer from the problem of variation in adhesion performance due to uneven crosslinking, and exhibits a good cohesive force even without performing a chemical crosslinking step. Further, this pressure sensitive adhesive has excellent reworking properties, adhesion properties, heat resistance, and durability, and can be very effectively used as a common pressure sensitive adhesive for an optical film. Moreover, the present inventors subsequently continued with their research into non-chemical crosslinking pressure sensitive adhesives for optical films, and found that when such an pressure sensitive adhesive for an optical film is used under harsher conditions, such as in an optical film that is used in a large-scale display, an automobile display or monitor, an outside display and the like, it would be desirable to further improve the adhesion properties and durability.
When peeling off an optical film for the purpose of recycling or to re-adhere the film, or when producing a liquid crystal display or a touch panel, static electricity can be produced. When static electricity is produced, problems can occur such as dust adhering to an optical part, abnormal display caused by disorientation of the liquid crystals, and electrostatic discharge failure of the peripheral circuit elements. From this perspective, to prevent problems caused by static electricity, attempts have been made in the past to impart an antistatic function to optical parts.
For example, it is known to bond an antistatic film via an pressure sensitive adhesive layer to a polarizing plate having a transparent conductive layer or a laminate formed from a polarizing plate having a transparent conductive layer and a retardation plate (refer to Patent Document 7). In this technique, the transparent conductive layer is formed from a transparent film formed by sputtering a conductive layer such as indium oxide/tin oxide, and an antistatic film is used in which the antistatic layer was formed by coating a surfactant.
In addition, a polarizing part having a quarter wave plate, a dichroic polarizing plate, and an antistatic layer on one or both sides of a Grandjean-oriented cholesteric liquid crystal layer is also known (refer to Patent Document 8). Here, the antistatic layer is formed by coating a UV-curable acrylic resin that contains metal oxide particles on the surface of the dichroic polarizing plate.
However, these conventional techniques require a separate step for forming an antistatic layer on an optical part. This increase in the number of steps causes problems such as a decline in productivity and increased costs.
Still further, an adhesive composition having antistatic properties formed by adding an ionic liquid to a polymer having a glass transition temperature Tg of 0° C. or less, and an pressure sensitive adhesive type optical part having an pressure sensitive adhesive layer formed from this adhesive composition on one or both sides of an optical part, are known (Patent Documents 9 and 10). However, this adhesive composition does not have sufficient adhesion to an optical film. Moreover, the adhered optical film or protective film may be peeled off and re-adhered or the optical film may be peeled off so that the expensive liquid crystal panel can be recovered and recycled. However, the reworking properties in recovering and recycling the expensive liquid crystal panel after the peeling-off of the optical film and durability cannot be said to be sufficient yet.